Search

As in many coastal environments, the bed of the Seine estuary is composed of mud and fine sand (respectively carried from the upstream river and from the sea). Their distributions are related to the hydrodynamic features of the estuary and the sediment bed can be stratified. In order to understand the major morphological behaviours of the estuary, a morphodynamic numerical process-based model is developed starting from a validated fine sediment transport model (Le Hir et al., 2001). The erodibility of the sediment accounts for the proportions of mud and sand; following Van Ledden (2001), two main regimes of erosion are distinguished (a non-cohesive and a cohesive one) using specific formulations to calculate the sediment erosion fluxes. In addition, the sediment column is discretized in thin layers (approximately in mm) and consolidation effects of mixed cohesive-type layers are simulated. Then the erodibility of the superficial layers depends on the relative mud concentration (i.e. between the sand grains). Simulations starting from arbitrary surficial sediment cover show that the model is able to reproduce realistic sediment distributions in the Seine estuary. The main observed morphological trends are qualitatively reproduced by the model, especially concerning the progradation and the rotation of the banks at the mouth. [ABSTRACT FROM AUTHOR]

Natural sediments are often mixtures of cohesive and non-cohesive sediment, and there are few data on the specific behaviour of these sediments, in particular, their erodibility. After a brief synthesis of recent works on the erosion threshold and the erosion rate of non-cohesive, cohesive and mixed sediments, the erodimetre, a new erosion device developed by IFREMER, is described. The instrument is portable and enables the separate quantification of the erosion of mud and sand fractions. Four data sets are presented, consisting of erosion tests on pure sands, laboratory mixtures with two types of mud and well-sorted sands, and natural mixed sediments. A clear relationship between the critical shear stress for erosion and the mud volume fraction (over the whole range) is shown. The correlation is not so good with the clay fraction. When the sand is fine (140 μm), the relationship is linear. When the sand size increases (280 μm), a sharp transition from non-cohesive to cohesive behaviour appears when the mud fraction exceeds 35-40%. It is suggested that the ratio between the grain sizes of the sand and the fine fractions, and more generally the whole size spectrum of the sediment, should be considered to characterise the sediment erodibility. This first interpretation extends the conceptual framework for the erosion behaviour described by Van Ledden et al. (2004). [ABSTRACT FROM AUTHOR]